1. Field of the Invention
The present invention relates to a calibration method suitable for application to measuring instruments accompanied by secular change in sensor output values such as zirconia-type oxygen analyzers.
2. Description of the Prior Art
FIG. 1 shows a cross-sectional view indicating the measurement principle of a zirconia-type sensor which is an example of sensors whose outputs vary (decrease) with time. In FIG. 1, sensor 1 is composed of zirconia tube 100 and electrodes 101 and 102 provided on the outer periphery and inner periphery thereof, and generally, porous platinum electrodes are used for electrodes 101 and 102. In zirconia-type sensor 1 having configured as described above, if a reference gas is passed along the outside of zirconia tube 100 (flow path of the reference gas) and a gas to be measured is passed along the inside of zirconia tube 100 (flow path of the gas to be measured) after heating the zirconia tube up to a high temperature of about 750° C., an EMF Vout corresponding to the difference of oxygen concentrations between the reference gas and the gas to be measured is generated between electrodes 101 and 102. The EMF Vout is proportional to the logarithm of the ratio of the oxygen concentrations, and the oxygen concentration of a gas to be measured can be determined from the value of the EMF Vout by using a gas whose oxygen concentration is known, such as air, as the reference gas.
FIG. 2 is a configuration drawing showing an example of conventional zirconia-type oxygen analyzers, which uses zirconia-type sensor 1. In FIG. 2, numeral 1 denotes a zirconia-type sensor as shown in the above indicated FIG. 1, numeral 2 denotes a converter which receives an EMF generated at zirconia-type sensor 1 and converts the EMF to a measured output signal corresponding to the oxygen concentration of a gas to be measured, numeral 3 a heater for heating up zirconia-type sensor 1, numeral 4 the heater power line supplying electric power to heater 3, numeral 5 the temperature control signal line for controlling the temperature of heater 3, numeral 6 the sensor output signal line, and numeral 7 the converter output signal line to transmit the measured output signal obtained by converter 2. Converter 2 also includes a temperature control system, not shown in the drawing, for holding the temperature of zirconia-type sensor 1 to a prescribed temperature.
Further, numeral 8 denotes a standard gas called “span gas” and numeral 9 denotes another standard gas called “zero gas,” and both standard gases are enclosed in each gas cylinder respectively. In general, “span gas” 8 is a gas containing 21% oxygen and 79% nitrogen, and “zero gas” 9 is a gas containing 1% oxygen and 99% nitrogen, and air is used for “span gas” 8. Numerals 10 to 13 denote control valves for switching the flow paths of gases which are supplied to zirconia-type sensor 1. Each operation of control valves 10 to 13 is controlled by converter 2.
In a zirconia-type oxygen analyzer configured as described above, at the time of measuring operations, “span gas” 8 is supplied to the flow path of the reference gas in zirconia-type sensor 1 through control valve 10, and at the same time, a sampled gas (gas supplied for the purpose of measurement) is selected by control valve 13 and supplied to the flow path of the gas to be measured in zirconia-type sensor 1 through control valve 11. As a result, an EMF corresponding to the difference of oxygen concentrations between the gas to be measured (sampled gas) and the reference gas (“span gas” 8) is generated from zirconia-type sensor 1. This EMF is converted to a converter output signal (4 to 20 mA) corresponding to an oxygen concentration of the gas to be measured (sampled gas) with converter 2 and the signal is transmitted via converter output signal line 7.
Next, at the time of calibrating operation, “span gas” 8 is supplied to the flow path of the reference gas in zirconia-type sensor 1 through control valve 10, and at the same time, control valve 13 is changed over to the calibration gas port and “span gas” 8 or “zero gas” 9 is selectively supplied to the flow path of the gas to be measured through control valves 11 to 13. That is, by changing over control valve 12 to “span gas” 8 port, “span gas” 8 is passed through the flow path of the gas to be measured of zirconia-type sensor 1 and thus calibration of converter 2 (span calibration) is performed for a sensor output corresponding to the oxygen concentration of “span gas” 8. Also, calibration of converter 2 (zero calibration) is performed for a sensor output corresponding to the oxygen concentration of “zero gas” 9 by changing over control valve 12 to “zero gas” 9 port to pass “zero gas” 9 through the flow path of the gas to be measured in zirconia-type sensor 1.                (Patent Document 1)        Gazette for Japanese Laid-open Patent Application No. 2000-266719        
Although the above mentioned calibrating operation is performed to output correct oxygen concentrations removing the influence of sensor deterioration and the like, clear standards are not shown for the timing to carry out calibrating operations and so in the current situation, it is up to the users to implement calibration periodically based on the user's experience.
For this reason, if the interval between calibrations is too long, the accuracy of measurements deteriorates and vice versa, if the interval is too short, calibrating operations take time and thus the working life of the measuring instrument is shortened. In addition, “zero gas” 9 is more expensive than “span gas” 8 which has the same constituents as air because the oxygen concentration of “zero gas” 9 is adjusted to a required value, and thus the more the number of calibrating operations increases, the more expensive standard gases are consumed.
The present invention aims at achieving a calibration method which removes the disadvantages in the above described conventional method, estimates the secular change of the sensor from the sensor output values at each calibration, and can determine the next appropriate calibration date, and also achieve zirconia-type oxygen analyzers using this method.